This is an interesting question. Scientists have found some evidence that indicates that the dinosaurs were extinct about 65 million year ago, at the end of the called Cretaceous Period of the Earth, which goes from 144 to 65 million year ago. At this time, a large amount of plant and animal families over about 55 pounds went extinct.

How do scientists know about the extinction of the dinosaurs? They know about because they have found chemical evidence showing that an asteroid of 4 to 9 miles in diameter hit the Earth. This impact probably penetrated the Earth's crust, scattering dust and debris into the atmosphere, and causing huge fires (generated by hot debris thrown from the crater), tsunamis (high sea waves), severe storms with high winds and highly acidic rain and seismic activity, perhaps even volcanic activity. Maybe the impact caused chemical changes in the Earth's atmosphere, increasing concentrations of sulfuric acid, nitric acid, and fluoride compounds. The heat from the impact's blast wave incinerated all the life forms in its path.

On the other hand, scientists have found bones of humans, indicating that the first human existed approximately 500,000 years ago. We do not know the name of these human beings, but we know that when the first human being appeared on Earth, the dinosaurs were already extinct. There are ways to know the age of a bone, by using radioactive Carbon 14. This radioactive form of Carbon is present in the atmosphere and all living things absorb it.

Then, according to the amount of Carbon 14 found on the bones, the scientists can determine the age of them.